. : ...... CHAPTEKXIX.
SECTION XlXa.—Influence of structure on the welding properties.—Wrought-iron may be welded so that the union is as strong as the rest of the bar, for by upsetting the piece there can be extra work put upon the metal, and since the strength of the original bar was dependent upon a great number of welds, the additional local heating and hammering may give a superior strength. Unfortunately, failure almost always takes place near the weld under destructive tests. A rod may break a short distance from the actual union, but this by no means shows perfect workmanship, for it arises from the overheating of the iron, without subsequent work to develop a proper structure.
In steel the conditions are different, for the bar is not a collection of fibers and welds, so that it is impossible to> make any improvement in a properly worked piece by cutting it in halves and putting it together again. It is conceivable that a bar may be underworked or overheated, and that additional work can enhance the strength, at the point of welding, but this assumption of a bad material to start with may be neglected. It is also possible to finish the hammering on a welded piece at a low temperature and thereby exalt the ultimate strength, but this will give a less ductile material.
It is also possible to have the weld stronger than the adjacent parts of the bars for steel will be crystallized by high heat more readily than wrought-iron, and hence it can happen that the metal in the neighborhood of the weld has a bad structure due to lack of hammering after high heating. The higher the critical temperature necessary to produce crystallization, the less the danger from this source, so that freedom from phosphorus and sulphur is a matter of importance.
The difference in crystallizing power between wrought-iron and steel makes a comparison of the two impossible, but it may be